The present invention relates generally to the welding of aluminum alloys by gas tungsten arc welding procedures and more particularly to the welding of such alloys when utilized as an overlay on metals damagable by the heat normally employed in the tungsten arc welding process.
Gas tungsten arc welding has been successfully utilized for welding various metals for a considerable period of time. In practicing the gas tungsten arc welding process, juxtaposed edges of the metal structures to be welded are placed together and the heat produced by the arc formed between a nonconsumable tungsten electrode and the abutting or fay surfaces of the metal structure causes the metal to melt or fuse to effect the weld joint. The weld zone, the fused metal and the arc are protected during the welding operation from contaminants in the atmosphere by effecting the welding operation in an inert gas environment. Normally, the welding temperatures employed in the gas-tungsten arc welding process are in a range of about 9,000.degree. to 12,000.degree. F.
Gas-tungsten arc welding processes have been used successfully for the welding of aluminum alloys especially structural aluminum alloys containing magnesium such as in the 5000 and 6000 series. For example, such welding processes are described in detail in the NASA Technical Brief B73-10481 entitled, "Welding High-Strength Aluminum Alloys", published Apr. 1974. In this publication, a considerable discussion is directed to the preparation of the aluminum alloys for welding as well as the various welding parameters used with gas-tungsten arc welding techniques. Inasmuch as the various alloys and the gas tungsten arc welding techniques discussed and utilized in this publication are relevant to the practice of the present invention this publication is incorporated herein by reference.
The techniques described in the aforementioned publication for the gas-tungsten arc welding of aluminum alloys provide full penetration welds which exhibit sufficient integrity for satisfactory use in many applications. Normally, the oxide layers on the fay surfaces are dislodged and float out of the weld at the high temperatures used during the welding process so as to minimize the porosity of the weld. However, in instances where the aluminum alloy is used as an overlay for protecting an underlying substrate formed of metal such as stainless steel, low-carbon steel or various cladding materials as used in reactor applications, from corrosive and other environmental conditions damaging to the substrate material, the practice of the gas tungsten arc welding processes as discussed in the aforementioned publication have not been particularly satisfactory. For example, in order to provide a successful weld of the aluminum alloy, sufficient heat must be utilized to dislodge the oxide layers on the fay surfaces to assure that no oxide inclusions or porosity due to the oxide is present in the weld joint. However, with the aluminum alloy disposed in an abutting or contiguous relationship with a substrate formed of stainless steel, low-carbon steel, or cladding materials for nuclear fuel elements, adequate heat cannot be applied from the arc to effect the dislodging of the oxides present on the fay surfaces as previously prepared without deleteriously damaging the substrate. Further, the application of sufficient heat to effect the dislodging of these oxides will not only damage the underlying substrate metal but will tend to pull some of the substrate metal into the weld joint so as to form undesirable inclusions of the substrate metal in the weld.
Inasmuch as the presence of the naturally occurring oxide layer on the fay surfaces of the aluminum alloy primarily responsible for the porosity in the weld and necessitated the requirement of the high heat input for the dislodging of the oxide layer, efforts were investigated for removing the oxide layer so that a lower heat input value may be utilized to successfully weld the aluminum alloy without thermally damaging the underlying substrate. The thickness of the oxide layer is directly proportional to the surface area on the fay surfaces. Conventional chemical and mechanical surface preparing and cleaning techniques such as described in the aforementioned publication were utilized for preparing the fay surfaces but it was found that these techniques did not sufficiently deplete the oxide layer so as to provide for a successful weld at the relatively low heat input values which would not damage the underlying substrate. FIG. 2 of the accompanying drawings is illustrative of such a weld where the heat input applied during a gas-tungsten arc welding operation is insufficient to deplete the oxides stringers in the weld joint when the fay surfaces were prepared by immersion in a warm solution of 72 vol. % phosphoric acid, 24 vol. % water and 4 vol. % nitric acid for 3 minutes and then rinsing in water.